Java Version set to 1.8

anonymous types replaced by lambda's
Method references used instead of Lambda's
Explicit type replaced by <>
travis.yml updated to java 1.8

mapdb_storage/src/main/java/io/moquette/persistence/mapdb/MapDBPersistentStore.java

@@ -83,13 +83,9 @@ public class MapDBPersistentStore implements IStore {
             m_db = DBMaker.newFileDB(tmpFile).make();
         }
         LOG.info("Scheduling MapDB commit task");
-        m_scheduler.scheduleWithFixedDelay(new Runnable() {
-
-            @Override
-            public void run() {
+        m_scheduler.scheduleWithFixedDelay(() -> {
                 LOG.debug("Committing to MapDB");
                 m_db.commit();
-            }
         }, this.m_autosaveInterval, this.m_autosaveInterval, TimeUnit.SECONDS);
 
         // TODO check m_db is valid and

mapdb_storage/src/main/java/io/moquette/persistence/mapdb/MapDBSessionsStore.java

@@ -191,7 +191,7 @@ class MapDBSessionsStore implements ISessionsStore, ISubscriptionsStore {
         Set<Integer> inFlightForClient = this.m_inFlightIds.get(clientID);
         if (inFlightForClient == null) {
             int nextPacketId = 1;
-            inFlightForClient = Collections.newSetFromMap(new ConcurrentHashMap<Integer, Boolean>());
+            inFlightForClient = Collections.newSetFromMap(new ConcurrentHashMap<>());
             inFlightForClient.add(nextPacketId);
             this.m_inFlightIds.put(clientID, inFlightForClient);
             return nextPacketId;